Conformal coating is the process of applying a dielectric material onto an electrical component, for example, a printed circuit (“PC”) board or a device mounted thereon, to protect it from moisture, fungus, dust, corrosion, abrasion, vibration, chemicals, tin whiskers and other environmental stresses. Conformal coating materials range from solvent based materials that cure by evaporation of the solvent, to “100% solids” conformal coating materials. Common conformal coating materials include silicones, acrylics, urethanes, epoxy synthetic resins, paralyne and various polymers. When applied to PC boards, an insulative resin film of uniform thickness is formed as a solvent evaporates or, as a solvent free material is cured.
Automated selective coating systems are known which have conformal coating dispensers that dispense material in various patterns, with varying deposition accuracies and producing coatings with varying thicknesses. For instance, a dispenser may dispense material in the form of a straight bead, a bead that is continuously rotated in a curved or circular pattern, and/or a bead that is subsequently atomized. Beads tend to produce coatings that are generally thicker than those for atomized sprays. Furthermore, depending on material viscosities as well as material/board surface tension interactions, a bead deposited on a board may spread to locations where no coating is desired. Moreover, in atomized sprays, injecting a supply of material with pressurized air to achieve atomization often creates significant overspray, thus depositing atomized droplets outside a target area.
These current dispensing methods have features that in some applications lead to undesirable coating results including greater than desirable minimum coating areas and less than desirable edge definition capability. With more recent conformal coating applications, it is desirable to have a capability of coating even smaller areas or smaller geometries. This capability, however, primarily depends on the type of dispenser used to apply the coating material and perhaps more specifically, the control a dispenser provides over the dispensed material.
With current dispensers that dispense beads or atomized sprays, there is a limit to which the size of the wetted area, or contact area of the bead or spray on a component, can be minimized. As a result, such current dispensers have minimum coating areas, i.e., an area where it is practical to use such a dispenser for conformal coating applications, which may be too large for more current applications. This becomes even more significant as boards and components get smaller and component densities on such boards increase.
Known needle valve dispensers control dispensing of a conformal coating material by controlling a time that a needle valve is open as well as an applied pressure of the conformal coating material being supplied to the needle valve. Such known needle valve dispensers also have some short comings, for example, first, it is difficult to control a flow of lower viscosity conformal coating materials. While there is no standard measure that distinguishes lower viscosity materials from higher viscosity materials, as an example, lower viscosity materials include but are not limited to materials that have viscosities that are less than one thousand centipoise. In order to reduce splash of lower viscosity materials through known needle valve dispensers, the dispensing tip must be maintained as close to the surface of the substrate as possible, for example, within several millimeters (“mm”). On PC boards that are densely populated, the requirement that the dispensing tip be maintained so close to the substrate surface may severely limit the application of known needle valve dispensers. Second, the inability of known needle valve dispensers to sharply cutoff a flow of conformal coating material results in dripping, drooling, poor flow control and generally, a less precise application. For example, the coating material can only be applied to within about 0.060 inch (“in”) or about 1.5 mm of a keep-out area, that is, an area where no conformal coating material is to be applied. Third, known needle valve dispensers have an inherent problem of allowing coating materials to cling to, and partially cure on, the dispensing tip, which leads to clogs and may reduce the repeatability and accuracy of the dispensing process.
Therefore, there is a need to provide an improved needle valve conformal coating applicator that improves the accuracy and selectivity of material deposition in a conformal coating process.